Conventionally, there have been measured circuit parameters (such as the S parameters) of a device under test (DUT) (refer to Patent Document 1: Japanese Laid-Open Patent Publication No. H11-38054, for example).
Specifically, a signal is transmitted from a signal source to a receiving unit via the DUT. The signal is received by the receiving unit. It is possible to acquire the S parameters and frequency characteristics of the DUT by measuring the signal received by the receiving unit.
On this occasion, measurement system errors are generated in the measurement due to mismatching between a measurement system such as the signal source and the DUT, and the like. These measurement system errors include Ed: error caused by the direction of bridges, Er: error caused by frequency tracking, and Es: error caused by source matching.
On this occasion, it is possible to correct the errors according to Patent Document 1, for example. The correction in this way is referred to as calibration. A brief description will now be given of the calibration. Calibration kits are connected to the signal source to realize three types of states: open, short-circuit, and load (standard load Z0). In these states, a signal reflected from the respective calibration kits is acquired by a bridge to obtain three types of the S parameter corresponding to the three types of state. The three types of variable Ed, Er, and Es are acquired from the three types of the S parameter, and then are corrected.
It should be noted that Er is represented as a product of an error Er1 relating to an input of a signal, and an error Er2 relating to a reflection of the signal. On this occasion, it is possible to connect a power meter to the signal source to measure the power, thereby measuring the errors Er1 and Er2 (refer to Patent Document 2: WO 2004/049564 pamphlet, for example).
The calibration described above can be applied to a switch-branch signal source. It should be noted that the switch-branch signal source is a combination of a signal source which generates a signal, and a switch which outputs the generated signal to any of multiple ports. If the above calibration is applied to a switch-branch signal source, the three states including the open state, the short-circuit state, and the load state (with the standard load of Z0) will be realized for the respective multiple ports, and a power meter is also connected according to necessity.
On this occasion, it is troublesome to measure Ed, Er1, Er2, and Es each time when circuit parameters of a DUT are measured. Therefore, it is desirable to record Ed, Er1, Er2, and Es measured at a certain time, and to correct circuit parameters of a DUT using the recorded Ed, Er1, Er2, and Es each time the circuit parameters are measured.
However, a secular change or a failure of a measurement system has possibly occurred between the time point when Ed, Er1, Er2, and Es were measured and the time point when the circuit parameters of the DUT are measured. A secular change or a failure of the measurement system may cause a change in Ed, Er1, Er2, and Es after they were measured. In this case, the correction using the recorded Ed, Er1, Er2, and Es cannot be precise correction.
When the circuit parameters of the DUT are measured, it can be determined whether Ed, Er1, Er2, and Es have been changed or not since they were measured by actually measuring Ed, Er1, Er2, and Es. However, in this case, it is not possible to avoid the trouble of actually measuring Ed, Er1, Er2, and Es.
It is an object of the present invention to easily carry out the calibration of a signal generation system such as a switch-branch signal source.